Beware of Eight Earthquake Hazard Zones

KOMPAS/ISMAIL ZAKARIA

Local residents and students, including those with disabilities, from 14 schools in Ulak Karang Utara and Ulak Karang Selatan villages take part in an earthquake drill on Saturday (2/09/2019) in North Padang district. The Padang municipal administration of West Sumatra province held the drill as part of disaster mitigation efforts to increase local preparedness in the event of an earthquake and tsunami disaster in Padang and other areas along the West Sumatra coast.

JAKARTA, KOMPAS — Despite the knowledge that the Sunda megathrust’s Mentawai segment is due for a major seismic event, disaster preparedness efforts should not be carried out in specific regions at the expense other regions. The government should also pay attention to other megathrust segments in the country, as these could also trigger major earthquakes at any time.

Seismologist Irwan Meilano of the earth sciences and engineering school at Bandung Institute of Technology (ITB) said on Sunday (2/10/2019) that global positioning system (GPS) monitoring showed that several tectonic zones in the country were due to release seismic energy at any time.

These tectonic zones lay beneath the Sunda Strait, southern West Java, Mentawai, Aceh Besar and Banda Aceh on Sumatra, the Maluku Sea and Banda Sea, as well as Sulawesi and Papua.

“There are many faults In Sulawesi and only the Palu Koro fault has partially released its energy. There are still the Matano and Lawanopo faults,” Irwan said. Even if the earthquakes caused by faults on land were relatively smaller, they could still be very destructive, as these faults were generally located near residential areas.

Meteorology, Climatology and Geophysics Agency (BMKG) head Dwikorita Karnawati said that eight earthquake hazard zones were currently under special observation. These zones had been deemed a threat because earthquakes rarely occurred along them, which meant that the faults held a relatively large amount of energy.

Aside from Mentawai, the other regions of concern are the Sunda Strait, the Kendeng fault, the subduction zone to the south of Sumba, the Banda Sea, western Lake Toba and the Flores back arc thrust, especially the segment north of Bali.

Recurring earthquakes

Indonesian Institute of Sciences (LIPI) seismologist Danny Hilman Natawidjaja said that the subduction zone beneath the Sunda Strait, which stretched all the way to the south of Bali, was also due for a seismic event. “Also, let’s not forget the earthquake zones in eastern Indonesia, which has triggered a major tsunami in the past in Ambon. It’s just that limited data is available compared to that on [earthquake zones] in western Sumatra, as limited research has been made on the area,” he said.

Danny said that data on recurring earthquakes to the west of Sumatra was understood, based on GPS and the Porites microatoll around the Mentawai Islands, Simeulue Island, Nias Island and other islands on Sumatra’s western coast. The Porites coral thrives in littoral zones.

He said that the growth of the Porites microatoll was highly influenced by changes in sea level, and could not grow higher than the sea level at low tide. The coral grew upwards until it reaches the water

surface. If an earthquake lifted the seabed beneath the microatoll, the part that became exposed above the sea would die, while the part below the sea would remain alive.

If the microatoll was lifted, the entire atoll would die. On the other hand, if the seabed subsided, the atoll would sink. The severity of submergence was measured by comparing the height of the microatoll and the height of low tide from the sea bottom following an earthquake. Danny and his team had found that islands off Sumatra’s western coast had undergone through several cycles of elevation and submergence.

“From the signs in the atoll, we found that earthquakes recur once every 200-300 years along the Mentawai segment. It has now been 222 years since the last major earthquake along the segment,” he explained.

Different from western Sumatra, earthquake studies are lacking in other regions in Indonesia. However, LIPI geotechnology head Eko Yulianto had made several paleotsunami studies and discovered recurring tsunamis in the southern waters of Java.

Potentially fatal

Danny said that in terms of risk, earthquakes caused by major subduction zones in the region could be fatal due to the high population density and economic activities around and in the Sunda Strait. Therefore, disaster preparedness efforts must also be carried out in other high-risk regions like it was in West Sumatra.

“Earthquakes are unpredictable by nature. Even Japan could not predict the 2011 earthquake and tsunami, as the segment that collapsed was not the one that they had been worried about. We need to watch out for the Mentawai segment, as it is only a matter of time [until a disaster occurs]. Nevertheless, we must not neglect earthquake hazards along other segments,” he said.

The major fault along the Mentawai segment could at any moment release an earthquake of up to magnitude 8.8 followed by a tsunami. The people in Mentawai Islands regency and Padang municipality must be prepared for the worst-case scenario. Danny said disaster preparedness could reduce the impact of a disaster.

At a coordination meeting on West Sumatra earthquake and tsunami mitigation in Padang, National Disaster Mitigation Agency (BNPB) chief Doni Monardo said the agency was paying serious attention to experts’ earthquake data and research. “Everyone must help one another and focus on reducing the risk of disaster in line with the President’s instruction,” he said (Kompas, Feb. 7, 2019). (AIK)